Inclusive Dielectron Production At （^SNN）^1/2=54.4 GeV Au-Au Collisions At RHIC-STAR

According to the Big Bang theory,there was a state of nuclear matter with extreme high temperature and high density within the first few microseconds after the Big Bang,the so-called Quark-gluon plasma(QGP).According to Quantum Chromodynamics(QCD)prediction,there exists a phase transition from the hadronic matter to Quark Gluon Plasma(QGP)at certain high temperatures and baryon density(μb)In the laboratory,QGP can be produced by relativistic heavy ion collisions.Relative Heavy Ion Collider(RHIC)at Brookhaven National Laboratory(BNL)is a facility used to produce QGP and study its properties.Dileptons(e+e-pairs&μ+μ-pairs)are excellent electromagnetic probes of QGP.Because they can be produced at all stages of the collision during the system evolution and do not interact with the medium strongly,dileptons can carry the information from the initial stage to the final stage.There are different physics interests in different mass regions for the dilepton spectral analyses.In the low mass region(LMR,Mee<Mφ),the main topic is the mass spectra of vector mesons modified by the hot and dense medium.This is related to the chiral symmetry restoration in the medium.In the intermediate mass region(IMR,Mφ<Mee<MJ/ψ),QGP thermal radiation can be used as a QGP thermometer.However,it is complicated to measure the QGP thermal radiation because of the heavy flavor semi-leptonic decay contributions.In the past decades,dileptons were measured by several experiments in different collision systems and collision energies at different facilities.The broadening of vector meson mass spectra was observed in the LMR.This broadening can be explained by several models with the scenario that chiral symmetry is restored in the QGP.In the IMR,models predict significant dilepton production from the QGP thermal radiation.However,It is hard to separate the thermal dileptons from the open heavy flavor decays at RHIC and LHC energies.At lower energies like the SPS energy regime,the thermal sources and open heavy flavor contributions can be adequately separated.NA60 experiment reports their temperature measurement result with high statistic and precision dimuon data in In-In collisions at(?)=17.3 GeV at SPS.The temperature NA60 measured is 246±15 MeV.The Beam Energy Scan program(BES)at RHIC was proposed to explore the QCD phase structure.The Beam Energy Scan Phase Ⅰ and Phase Ⅱ took data at different collision energies from(?)=200 to 7.7 GeV.As a part of the Beam Energy Scan program,The Solenoidal Tracker at RHIC(STAR)collected 1.2B Au+Au collision events with minimum bias(MB)trigger at(?)=54.4 GeV at 2017.With high statistics compared to BES-I datasets and lower heavy flavor semi-leptonic decay contributions compared to those at RHIC top energies,QGP thermal radiation in the IMR will be measured for the first time at RHIC.In this thesis,we reported the measurement of dielectron production in Au+Au collisions at(?)=54.4 GeV at STAR.This measurement reports the first temperature measurements via the thermal dielectrons emitted from the hot and dense QCD matter of both the hadronic and partonic phases created in Au+Au collisions at RHIC.The temperatures measured in the low mass region in all centrality bins are close to the critical temperature predicted by the Lattice QCD.That provides the first direct experimental evidence that the in-medium p is predominantly produced around the QCD phase transition from QGP matter to hadronic matter at RHIC.In the intermediate mass region,the temperature extracted from(?)=54.4 GeV Au+Au collisions in 0-80%centrality is 338 ± 58 MeV,significantly higher than the predicted QCD phase transition temperature and temperature extracted from the hadronic chemical distribution.This is consistent with the expectation that the dominant emitting source is the deconfined QCD matter(QGP).This is the first direct temperature measurement of QGP created in the heavy-ion collisions at RHIC.